Functional analysis of GC Box and its CpG methylation in the regulation of CYP1A2 gene expression

Although there is a putative GC box in the cytochrome P450 1A2 (CYP1A2) promoter, its function has remained undetermined. To understand the molecular mechanisms controlling CYP1A2 gene expression in the liver, we studied the roles of the GC box in promoter activity and the effects of its CpG methylation on CYP1A2 gene expression. The results of luciferase assays showed that promoter activity was significantly dependent on the presence of the intact GC box. The results of bisulfate sequencing showed that the CpG methylation status of the GC box was strongly associated with CYP1A2 mRNA expression in human cell lines and tissues, suggesting that CpG methylation is involved in the tissue-specific regulation of CYP1A2 gene expression. However, effects of in vitro CpG methylation of the GC box on the promoter activity were not so dramatic in the luciferase assay, suggesting that the major function of the methylated-CpG is not to inhibit transcription factors in binding to the GC box. Taken together, our results show that the GC box is a critical element for the CYP1A2 promoter and its epigenetic regulation mediated by CpG methylation may play important roles in the tissue-specific CYP1A2 gene expression. Genome-based approaches may be necessary for understanding this tissue-specific epigenetic mechanism.